Abstract: Inertial sensors of micro electro mechanical system(MEMS) are a kind of sensors transforming physical quantity of motion into electrical signal. Their working principles include the disciplines of physics, mechanics, microelectronics and so on. With the continuous size reduction and performance improvement of MEMS chips, traditional semiconductor failure analysis methods can hardly meet the demand of MEMS device failure analysis. In this paper, the working principles of MEMS devices and focused ion beam(FIB) dual beam system were discussed, and three common failure phenomena of MEMS chips were analyzed by focusing on FIB dual beam system, namely current leakage, abnormal sensitivity and zero input offset drift. It is found that the common failure modes of MEMS chips include electrode structure damage, silicon particle inducing during wafer manufacturing, and so on. During this process, the flexibility and superiority of FIB dual beam system in MEMS failure analysis are presented.